1. Field of the Invention
This invention relates to a technique for imparting a tint to, for example, a gray image for printing.
2. Description of the Related Art
FIG. 22 is a block diagram showing conceptually a technique for printing a color image using a color printer. Image data DT2, indicating an image scanned by a scanner 20, is output to a computer 10. Based on the image data DT2, computer 10 displayer the image on a CRT 22, and sends the image to a color printer 30 for printing. If it is desired to print the scanned image in color, image data DT2 is composed of an R signal, a G signal, and a B signal (hereinafter collectively termed “RGB signal”) that indicate amounts of red, green, and blue, respectively.
An application program 40 runs on a predetermined operating system on computer 10. This operating system includes CRT driver software 17 and printer driver software 14. Image data DT1 for transfer to color printer 30 is output by application program 40, through the printer driver software 14.
Application program 40 consists, for example, of photo retouching software, and is used to perform processes such as image retouching on image data DT2. The process result DT3 obtained from application program 40 is provided to CRT driver software 17 and printer driver software 14.
When application program 40 issues a Print command, the printer driver software 14 on computer 10 converts the process result DT3 into a print signal DT1 and sends this to the color printer 30. Color printer 30 is provided with various types of ink; DT1 includes data indicating dot formation status (dot data) for multiple types of ink, and information regarding sub-scan feed distances.
Printer driver software 14 includes a resolution conversion module 41a, color conversion module 41b, color conversion table 41e, halftone module 41c, and rasterizer 41d. 
Resolution conversion module 41a converts the resolution of the process result DT3 received from application program 40 to the printing resolution, to obtain conversion result DT4. Naturally, conversion result DT4 also contains information regarding color. Using the color conversion table 41e, color conversion module 41b determines, on a pixel-by-pixel basis based on conversion result DT4, the ejection amount of each type of ink that will be used by color printer 30. Halftone module 41c performs a so-called halftone process. Rasterizer 41d arranges dot data in the order in which data will be transmitted to color printer 30, and outputs the final print data in the form of print signal DT1 to color printer 30.
This technique has been described, for example, in JP2002-59571A. A technique for printing a plurality of images corresponding to a plurality of colors using a color printer has been described, for example, in JP11-196285A.
Techniques for representing color images on printing media in this way are widely used. However, monochrome images having a single tint (also termed “monotone images”) can have a unique ambience when having a certain tone, and thus demand for printing monochrome images is high as well. The conventional technique illustrated in FIG. 22 can be used for printing of monochrome images as well.
For example, an image scanned by scanner 20 is recognized by computer 10 to be a colorless gray image. Since, in a gray image, each pixel has equal amounts of red, green, and blue, the R signal, G signal, and B signal of image data DT2 assume mutually equal values.
Application program 40 performs a process for imparting predetermined tint to the gray image represented by image data DT2 (hereinafter termed “tinting process”) to produce process result DT3.
FIGS. 23 to 26 are graphs illustrating RGB signal conversion associated with the tinting process; the new R signal, G signal, and B signal belonging to process result DT3 obtained by the tinting process are designated respectively as the R′ signal, G′ signal, and B′ signal (hereinafter collectively termed “R′G′B′ signal”). The R signal, G signal, and B signal of image data DT2 assume mutually equal values. In the example given here, the RGB signal can assume 256 tone levels, corresponding to tone values which are integers from 0 to 255.
FIG. 23 illustrates an instance in which it is desired to print a gray image as a gray image (hereinafter termed “neutral tone”); FIG. 24 shows an instance in which it is desired to print a cool image (hereinafter termed “cool tone”); FIG. 25 shows an instance in which it is desired to print a warm image (hereinafter termed “warm tone”); and FIG. 26 shows an instance in which it is desired to print a color shade resembling a discolored color photograph (hereinafter termed “sepia tone”), respectively.
Once an R′G′B′ signal obtained in this manner has been converted in resolution by resolution conversion module 41a, in color conversion module 41b, it is converted, with reference to the color conversion table 41e, into ejection amounts of the various types of ink used by color printer 30. Values of the R′G′B′ signal are retained even after resolution conversion by the resolution conversion module 41a. 
FIG. 27 is a graph describing a technique for setting ejection amounts C, M, Y, K for cyan, magenta, yellow, and black inks on the basis of an R′G′B′ signal, using color conversion table 41e. Since the R′ signal, G′ signal, and B′ signal are mutually independent, color conversion table 41e is represented conceptually as a three-dimensional cube. Here, 256 (=28) tone levels with tone values of 0 to 255 are shown. From the standpoint of limited memory capacity, it is undesirable for color conversion table 41e to store 28×28×28 independent sets (approximately 16,780,000 sets) of data. Thus, data memory locations in color conversion table 41e are set up discretely as grid points, in sets of 17 tone values, for example. Here, a single set of data includes, for example, three kinds of data for ink ejection amounts C, M, Y. FIG. 27 shows as an example a location T0 at which the R′ signal, G′ signal, and B′ signal assume values of r0, g0, and b0 respectively.
However, there are typically instances in which grid points corresponding to arbitrary values r0, g0, and b0 do not exist. In such instances, it is typical to select several grid points around location T0, and through interpolation using ink ejection amounts in memory for the selected grid points, set an ink amount corresponding to location T0.
In the arrangement described above, the process of setting a tone of a monochrome image to desired tone is not easy. In particular, despite the fact that 256 types of RGB signal are sufficient in image data DT2 representing a gray image, as the R′ signal, G′ signal, and B′ signal representing a monochrome image assume different values, a color conversion process similar to that used for a color image is required when printing the monochrome image. Additionally, setting a tone in a monochrome image involves trial and error, increasing the time required for the process.